This invention pertains generally to optical fibers and more particularly to optical fibers with reduced temperature sensitivity.
An optical fiber is generally a composite construction comprising a light-propagating core, at least one outer layer, often referred to as cladding, one of which has an index of refraction lower than that of the core, and usually one or two protective coatings. If two or more outer layers are used, the last layer is often referred to as the substrate. The protective coating is usually plastic or metal. For example U.S. Pat. No. 4,213,672 by Aulich et al. discloses a single and a double plastic protective coating.
Performance of an optical fiber is improved by either improving the properties of the core or one of the layers or making certain matches of properties between the core and the different layers. Two examples of borosilicate optical fibers having variations in compositions to improve the optical fiber. are given. In U.S. Pat. No. 4,089,586 by French et al., the ratio of SiO.sub.2 and B.sub.2 O.sub.3 in the core and cladding is varied, but is never less than 3:1, to produce a single mode optical transmission. A multi ingredient borosilicate glass having no more than 15 weight percent of B.sub.2 O.sub.3 is disclosed in U.S. Pat. No. 4,265,667 by Ikeda et al.
Temperature response of optical materials has been studied and researched for many years, but recently the research has increased significantly. The following are examples of recent work. In Roy M. Waxler and G. W. Cleak "The Effect of Temperature and Pressure on the Refractive Index of Some Oxide Glasses" J. Res. NBS 77A-6, p. 755-63, Nov.-Dec., 1973, the effect of temperature on the index of refraction is noted and the resulting adverse effect on light scattering is noted. However, no solutions to this problem are given. An earlier study on the effect of temperature and photoelastic constants on the index of refraction is given in R. M. Waxler et al. "Optical and Mechanical Properties of Some Neodymium-Doped Laser Glasses" J. Res. NBS 75A:3, p. 163-74, May-June, 1971. A mismatch between a borosilicate elliptical cladding and a pure silica outerjacket is reported in, V. Ramaswamy et al. "Birefringence in Elliptically Clad Borosilicate Single-Mode Fibers" App. Opt. 18:24, p. 4080-4, Dec. 15, 1979, to cause stress-induced strain birefringence.
Only until recently has much research been undertaken concerning the temperature sensitivity of the phase of light propagating in an optical fiber. An example of this work is N. Lagakos et al. "Temperature-Induced Optical Phase Shifts in Fibers" App. Optics 20:13, p. 2305-8, July 1, 1981, which discusses maximizing the temperature sensitivity of the fibers.
This sensitivity can be an important consideration in optical fiber design. In communication systems, changes in phase can deteriorate the signal in high-bandwidth communications. Temperature-induced phase shifts add to the noise of fiber sensors, e.g. acoustic, magnetic, and gyros. Similar problems exist with fiber-optic temperature sensors in that the sensing fiber should be temperature sensitive, but the optical fiber lead and reference fiber should not be.